1. Field of the Invention
The present invention relates to nitride semiconductor devices applicable to power transistors for use in power supply circuits of consumer products, such as television sets.
2. Description of Related Art
In recent years, active research and development have been made on the application of field effect transistors (hereinafter referred to as FETs) using a GaN-based material to high-frequency and high-power devices. Nitride semiconductor materials such as GaN are capable of producing various kinds of mixed crystal such as AlN and InN, as well as heterojunction just like conventionally used arsenic semiconductor materials such as GaAs. At the heterojunction interface between nitride semiconductor layers, high concentration carriers are generated by spontaneous or piezoelectric polarization without injecting any dopants. Therefore, if the nitride semiconductor material is used, the resulting FET is likely to be a depletion mode (normally-on) FET and it is difficult to obtain an enhancement mode (normally-off) FET.
FIG. 8 is a sectional view illustrating a conventional FET having an AlGaN/GaN heterostructure.
In the conventional FET shown in FIG. 8, a low-temperature GaN buffer layer 1802, an undoped GaN layer 1803 and an n-AlGaN layer 1804 are formed on a sapphire substrate 1801 in this order. A source electrode 1805 and a drain electrode 1806 made of a Ti layer and an Al layer are formed on the n-AlGaN layer 1804. A gate electrode 1807 made of a Ni layer, a Pt layer and an Au layer is formed between the source and drain electrodes 1805 and 1806. A SiN film 1808 is formed as a passivation film. This FET is a normally-on FET in which drain current flows when a gate voltage is 0 V due to high concentration two-dimensional electron gas generated at the heterointerface between the undoped GaN layer 1803 and the n-AlGaN layer 1804.
In the current power electronics market, however, most of the devices are normally-off and devices using the nitride semiconductor materials such as GaN are also required to be normally-off. To meet the requirement, a junction field effect transistor (JFET) using a pn junction for the gate in a conventional GaAs compound semiconductor has been proposed and put into actual use (see J. K. Abrokwah et al., IEEE Transactions on Electron Devices, vol. 37, no. 6, pp. 1529-1531, 1990). When the JFET structure is adopted, a pn junction having a higher built-in potential than that of a Schottky junction is used for the gate to increase a gate rise voltage (a voltage at which the gate current is started to flow), thereby reducing gate leakage current. According to a recent report, the JFET structure has been used also in a nitride semiconductor (see L. Zhang et al., IEEE Transactions on Electron Devices, vol. 47, no. 3, pp. 507-511, 2000 and Japanese Unexamined Patent Publication No. 2004-273486).